Survival and Natal Dispersal of Fledglings of Tengmalm's Owl in Relation to Fluctuating Food Conditions and Hatching Date

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Abstract
(1) 4311 fledglings of Tengmalm''s owl were ringed during 1964-85 in western Finland: 53 (1.2%) were later recovered after surviving at least the first winter. (2) The proportion of fledglings which were recovered was significantly larger in the increase phase of the vole cycle than in the peak, decrease and low phases. The numbers of fledglings produced were larger in the increase and peak phases than in the decrease and low phases. Thus, the number of fledglings surviving in the increase phase was twice as large as that in the other phases. There were no differences in the distances of natal dispersal between the four phases of the vole cycle. (3) Hatching date within the season did not seem to affect survival. Because early clutches produced more fledglings than late ones, clutches hatched in April produced 1.5 times as many surviving young as those hatched in May-July. Distances of natal dispersal of early and late young were nearly similar. (4) Food conditions during the post-fledging and independence periods seemed to be crucial for the survival of the young. Because vole cycles ware fairly predictable, a selective advantage could be expected for those owls that invest most in reproduction during the increase phase, the contribution to the future gene pool of the population being highest at that time. (5) Because the survival of the young varies in the course of the vole cycle, the reproductive value of an egg depends both on laying date and food supply.